It is known that a non-electroconductive material, such as plastics, may be made electroconductive by the addition of an electrically conductive powder. Examples of known electroconductive powders include metal powders, carbon black, and tin oxide doped with antimony or a like dopant. Addition of metal powder or carbon black to plastics makes the plastics black, which can limit the utility of the plastics. Addition of tin oxide doped with antimony, etc. makes plastics bluish black, which can limit the utility of the plastics as with the case of adding carbon black. In addition, using antimony involves the problem of environmental burdens. Hence, various studies have been reported on tin oxide free from a dopant, such as antimony.
Patent literature 1 (see below) describes alkali-stabilized tin oxide sol having a particle size of 30 nm or smaller and containing tetramethylammonium hydroxide in an NH3 to SnO2 molar ratio of 0.01 to 0.3. The tin oxide sol is obtained by adding tetramethylammonium hydroxide to an alkaline tin oxide sol having a tin oxide concentration of 15 wt % or less in terms of SnO2, followed by concentration.
Patent literature 2 teaches an alternative process for preparing tin oxide sol comprising adding tin to 0.1 to 8 N hydrochloric acid in an HCl to Sn molar ratio of 0.5 to 1 and then adding thereto a hydrogen peroxide solution. According to the disclosure, the resulting tin oxide particles have an average particle size of 5 to 100 nm.
Patent literature 3 proposes particles, which are not tin oxide particles but a precursor for producing tin oxide particles, having a flaky shape and containing 60 to 88 wt % of Sn and 1 to 15 wt % of organic matter in terms of carbon. The precursor particles disclosed show a sharp peak at about 9° in XRD. According to the disclosure, this peak is attributed to the flaky shape of the particles.
However, the tin oxide particles produced by the above described techniques are not regarded as being sufficient in electroconductivity and transparency when formed into film.
Apart from the above techniques, tin (II) oxide having an orthorhombic crystal structure with lattice constants of a=0.5 nm, b=0.572 nm, and c=0.1112 nm is reported in non-patent literature 1 (see below). A report on the space group of this tin oxide is also found in the same literature. Based on these data, the inventors of the present invention calculated the X-ray diffraction peak of the tin oxide and found that the peak is at about 28°. It was also found that the tin oxide shows a peak ascribed to the internal structure at about 60° or greater. According to the literature, however, the tin oxide is unstable and ready to change to another structure. The literature is silent on the electroconductivity or transparency of the tin oxide.